Non-poisoning mass spectrometer



1956 J. E. CLEMENS ETAL 2,770,735

NONPOISONING MASS SPECTROMETER Filed May 9, 1955 INVENTORS JOHN E. CLEMENS Y BEN B. JOHNSTONE United States Patent NON-POISONING MASS SPECTROMETER John E. Clemens, near Xenia, and Ben B. Johnstone, Dayton, Ohio Application May 9, 1955, Serial No. 506,828

Claims. (Cl. 250-413) This invention is concerned with increasing the accuracy of gas analysis in a mass spectrometer.

A mass spectrometer is an instrument widely used for the accurate analysis of gases. The analysis is executed by first ionizing the gas to be analyzed and then effecting a separation of the ions produced by utilizing the difference in their mass to charge ratios. In order to ionize the gas, an electron source is provided which with electron accelerating means produces a high velocity stream of electrons. The electron stream is directed through the gas so as to bombard the gas molecules, thereby converting them into ions.

The accuracy of the analysis of gases in mass spectrometers using this method of ionization is not sufficient for many applications due to the oxidation or poisoning of such gases effected within the instrument itself. Such oxidation is highly objectionable since the results obtained are incorrect in proportion to the amount of oxidation occuring Within the spectrometer. This poisoning is promoted by direct contact of the gas with the hot electron source used for the production of the electrons with which the gas is bombarded. Except for this disadvantage, the method of ionization described is completely satisfactory.

The present invention consists in isolating the heated electron source which may be a heated filament, in a mass spectrometer from the gas to be ionized by interposing a thin window which is permeable to electrons but impervious to gas molecules. In this way substantial improvement in accuracy is attained. The improvement may be embodied in any type of mass spectrometer having a heated electron source.

The invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawing, in which Fig. 1 is a sectional expanded View of a dual section mass spectrometer which includes the aforementioned invention.

Fig. 2 is a sectional view of a modification of the invention illustrated in Fig. 1.

Fig. 3 is a sectional view of a single section mass spectrometer employing magnetic deflection and incorporating the aforementioned invention.

The mass spectrometer illustrated in Fig. 1 is composed of two separable sections. The two sections are shown divided from one another. In actual operation, section 1 is inserted into section 20, thereby forming a single unit. Section 1 is a gas tight chamber in which the elec tron source is enclosed. Section 20 is a second chamber containing the gas to be ionized and means for ion analysis and collection. The separability of the two chambers makes repair and replacement of any of the contents of either chamber convenient.

Section 1 is composed of glass cylinders 2 and 12. The walls of cylinder 2 are thickened locally to provide joint surface 3 and rear support 4. Leads 5 and 6 conduct heating current to the electron source 9, which in this case is a directly heated filament. Leads 7 and 8 estab- 2,770,735 Patented Nov. 13, 1956 ice lish a difference of potential between filament 9 and grid 10 located adjacent to a Lenard type window 11. The potential dilference is such that electrons emitted from filament 9 are accelerated by grid 10 through the window 11. Cylinder 12 is provided so that evacuation of chamber 1 may be effected thereby permitting electron flow unimpeded by gas molecules.

A Lenard type window has the characteristic of being electron permeable while impervious to gas molecules. When mounted in a gas-tight manner .in the walls of cylinder 2, in the path of the electron stream produced by filament 9 and grid 10, it permits the penetration of electrons into chamber 20, but blocks the passage of gas from chamber 20 into gastight chamber 1, thereby preventing direct contact between the gas and the electron source.

The Lenard type window may be made of aluminum or beryllium. If made of these materials, it should not vary more than 0.00005" from a median of 0.0002. Mica, if pure and strong, may also be used, and should be about 0.02" in diameter. It is the generally preferred material, although for special uses the other materials mentioned may be more advantageous. For example, beryllium is preferred if a supply of neutrons is desired. In order to insure that the window is gastight around its perimeter, it is fused to the glass wall of cylinder 2.

Section 20 is a glass chamber housing a conventional mass spectrometer with the exception that the electron source used for ionization of the gas is not present. It is composed of three glass cylinders 21, 22 and 23. Cylinders 22 and 23 are provided to permit the admission and ejection of the gas to be analyzed. Grids 24, 25 and 26 compose an ion cage. The positive potential applied to these grids further accelerates the electron stream which originates in chamber 1 and passes through window 11 into chamber 20. The electron stream passes through the admitted gas, and bombards the gas molecules thereby causing ionization. Grids :27 and 28 have a potential of the appropriate polarity and strength to attract the ions formed in the ion cage and propel them into the analyzing section indicated by the dotted enclosure 30. The operation of the analyzing section of this mass spectrometer is of no importance with respect to the invention described. The analyzing section 30 therefore may incorporate any of the well known methods of ion selection used in mass spectrometers. The selected ion mass is then collected at ion collector 31.

Chamber 20 is provided with a neck 32 whose inner surface 33 is ground to fit the thickened portion 3 of cylinder 2 thereby providing a gastight fitting.

It may be advantageous to have the Lenard type window mounted in the wall of the gas containing chamber rather than that of the chamber enclosing the electron source as indicated in Fig. 2, wherein numerals employed heretofore are used to identify similar elements. The Lenard type Window 11 is fused into the walls of cylinder 21 while cylinder 2 containing the electron source has an unobstructed opening 40. This arrangement permits replacement of the filament without the necessity of removing the Lenard type Window fused into position.

Fig. 3 illustrates the invention incorporated in a single section mass spectrometer which employs magnetic deflection to effect the desired ion separation. The apparatus includes a tube or vessel 51 dispersed within an electromagnet 52. The vessel 51 can be made of glass in which case it is provided with a grounded metallic lining or coating 53 in the usual manner. If desired, however, the vessel may be made entirely of metal in which case no separate lining is required. The particular structure of the electromagnet 52 should be such as to provide a uniform magnetic field throughout the tube perpendicular to the plane of its curved portion. Tube or pipe 54 admits gas'int'o the vessel 51 through opening 55. 'The tube -is evacuated through "an opening 56 at approximately the center thereof. Collector plate 57 is located at one end of the tube with battle plate 58 before it. The ioni'z'in'gmeans ishousedat the'other end of the tube. The electron source 61 and electron accele'r'ating grid are located 'in char nber 60. The electron source is "sealed "Within this chamber by the Lenard type window '63. The gas introduced through .inlet '55 is therebyde'nied'access to the electron Source. However, the electron stream formed by elements 61 and '62 can pa'ssahrough window"63 to electron collector 64 thereby passing through the introduced gas. The bombardment of the gas molecules by the electron stream causes the desired ionization. The ionsare then accelerated "by ion accelerating electi'o'des "65 and 66 and separation is effected in "their passage through the'tube.

The'd'e'scribed mass "spectrometers arefr'ee of the inacciiracy in analysis found in prior instruments of this sort. The use of a Lenard type window mounted in a gas tight manner between the electron source and the gas to be ionized prevents direct contact between the two thereby avoiding'the condition which promotes the oxidation of the-gas under analysis.

While specific embodiments have been described to illustrate the invention, it will be evident that modifications may be made without departing from the scope of the invention as set forth in the appended claims.

We claim: 7

l A mass spectrometer having a first chamber; an electrons'ource within said chamber; a second chamber adapted to enclose a gas to be ionized'by electrons from said source; and a window permeable to electrons but impervious to gas positioned'between the chambers thereby separating the electron source from the gas to be ionized.

2. A mass spectrometer having a first chamber; an electron source within said chamber; a second chamber adapted to enclose a 'gas to be ionized by electrons from said source; and a Lenard type window positioned between the chambers thereby separating the electron source from th'e'ga's to be'ionized.

3. A mass spectrometer'according to claim 2 in which the Lenard type window is aluminum foil.

4. A mass spectrometer according to claim 2 in which the-Lenard type window is beryllium foil.

5. A mass spectrometer according to claim 2 in which the Lenard type window is mica;

6. A mass spectrometer comprising an evacuated chamber; an electron gun within said chamber adapted to produce a stream of electrons; a second chamber adapted to enclose a gas to be ionized by electrons of said stream; and a Lenard type window positioned between the chambers in the path of said electron stream thereby separating the electron source from the gas to be ionized.

7. A mass spectrometer comprising an evacuated chamber; an electron gun within'sai'cl chamber adapted to produce a stream of electrons; a second chamber adapted to enclose a gas 'to' be ionized by electrons of said stream; and a Lenard type window arranged to form a v gastight wall common to both chambers.

8. A mass spectrometer comprising an evacuated chamber; an electron source and an electron accelerator within said chamber, the combination producing a stream of electrons; a second chamber adapted to enclose a gas to be ionized by electrons of said stream; and a Lenard t pe windew positioned in and forming part of a gastig'ht partition separating the two chambers, said window being in thepath of the electron stream.

9. A mass spectrometer comprising a first envelope; an electron gun within said envelope adapted to produce a stream of electrons; an opening in said envelope adapted to permit evacuation of the envelope; a Lenard type window arranged in said envelope in a gas tight manner and positioned "in the path of the electron stream;ja second envelope adapted to enclose a gas to be ionized by electronsof said stream; and means for detachably afiixing the 'first envelope to the second envelope in a gastight manner;

I0. A "n"1 ass spectrometer comprising a first envelope; an "electron gun within said envelope adapted to produce a stream of el'ceens'; an opening in said envelope adapted to permit evacuation of the envelope; 2. second envelope adapted to enclose a gas to be ionized by electrons of said stream; 'a Lenard type window arranged in said en;

velope in gastight m anner and positionedin the path of said stream; and means for detachably affixing the first envelope to the second envelope in a 'gastight manner.

No references cited. 

